Add support for draft-ietf-idr-ext-opt-param

As BGPCEP-867 shows, we can easily exhaust Optional Parameters field size. There is an old draft, https://datatracker.ietf.org/doc/draft-ietf-idr-ext-opt-param/ , which proposes an extension to the size field, so that we can advertise longer parameters.

The draft seems to be deployed (https://mailarchive.ietf.org/arch/msg/idr/FvW4WnO8loRe6dQNq-pW-Tfjn3w ), so we do have a chance to interop with at least some implementations, even when ExaBGP does not support it.